The object of the present invention is a device for the measurement, at a plurality of points, of the microwave field radiated by a source, comprising:
at each of said points, at least one antenna, loaded by at least one diode, PA1 means for collecting the microwave radiation coming from each of said antennas, delivering a collected microwave signal, PA1 means for generating a low-frequency signal, PA1 multiplexing means, arranged between said generator means and each of said diodes, PA1 an addressing circuit connected, on the one hand, to said multiplexer and, on the other hand, to each of the diodes, and PA1 means for controlling the said multiplexer means in such a manner that at least one of said diodes is polarized by said low-frequency signal and in order, in response to said low-frequency signal and to said collected microwave signal, to generate a signal representative of the microwave field at the point where said antenna charged by said polarized diode is located. PA1 the said antennas are distributed uniformly over a surface, and PA1 the said source and the said collecting means are disposed on the same side of said surface and in the vicinity of each other so that the path from said source towards said surface and the path from said surface towards said collecting means are close to each other. PA1 a layer of material is provided which absorbs the microwave radiation and the shape of one face of which fits said surface, the layer being arranged on the side of said surface opposite that of said source and of said collector means, and PA1 the said addressing circuit fits in shape the other face of said layer and is connected to each of said diodes via connections which pass through said layer perpendicular to said surface.
Such a device makes it possible, for instance, when the source is an antenna to be characterized, to draw up, by the method known to the person skilled in the art as the "modulated diffusion method", a map of the field radiated by the antenna. It may also be used when the source, instead of being an antenna which is to be characterized is an object diffracting a microwave field produced by an auxiliary antenna the radiation pattern of which is known. One can thus, in non-destructive manner, check industrial objects or products or else study the diffraction by a body of complicated shape or composition.
From French Applications 2 614 416 and 2 632 417 devices of the above-defined type are known in which the measurement points are aligned.
These devices are therefore suitable for the carrying out, without mechanical displacement, of one-dimensional measurements of the microwave field, that is to say, along the line or curve on which the antennas are aligned, by the electronic scanning of them.
However, these devices are not suitable for the carrying out, without mechanical displacement, of two-dimensional measurements of the field so as to know the field on a surface.
Furthermore, these one-dimensional devices cannot be simply transposed to two-dimensional devices, due to the presence of the connections which connect each of the diodes to the multiplexer and which it is necessary to make invisible vis a vis the microwave field, in order not to disturb it. Now, while it is relatively simple to make these connections invisible when the antennas are aligned, as shown by the solutions described in the aforementioned applications 2 614 419 and 2 632 417, it is much more difficult to do so in the event that the antennas are distributed over a surface, without limiting the pass band and possibly the sensitivity of the device.
Furthermore, when it is desired to measure, at each point, the components of the field along two perpendicular directions of polarization, it is necessary to have at each of the points two doublet antennas which are perpendicular to each other, for instance. This considerably complicates the preceding problem related to the connections for the feeding of the low-frequency signal towards each of the antennas.
Furthermore, in the device described in Application 2 614 419, the network of antennas is arranged in the proximity of a microwave collecting antenna, comprising for instance a waveguide having a series of slits. From this there results a relatively complicated construction and a risk of interaction between the object being tested and the detection unit formed by the antenna network and the collector waveguide.
These problems do not arise in the device described in Application 2 632 417 due to the fact that the linear network is arranged at the focal point of a reflector, itself arranged at a distance from the path of radiation coming from the source. Due to the focussing employed, such a solution, however, cannot be transferred to a two-dimensional network of antennas.
Furthermore, from French Patent 2 509 064 a device is known which permits the formation of a two-dimensional image of a microwave field, known accordingly as a "microwave camera." This camera employs a two-dimensional network of doublet antennas, for which the problem related to the feed connections of the low-frequency modulation signal is controlled in particular by the use, immediately behind the network of antennas, of a plurality of microwave collector antennas in the form of horns, stacked one on the other and multiplexed. This solution, which employs a large number of horns and a microwave multiplexing is, however, complicated and of relatively high expense. Furthermore, it does not control the problem of the measurement, without mechanical displacement, of the components of the field along two perpendicular directions of polarization, or that related to the interaction with the antenna or object being tested.
From European Patent Application 0 065 455 a device of the type defined above is also known, in which:
In such a device, due to the fact that the antennas are distributed over a surface, it is possible to carry out two-dimensional measurements without moving the test object relative to the measuring device, by a simple electronic scanning of the network of antennas.
The addressing circuit, however, has the drawback that the collecting means are visible and therefore extensively disturb the measurements, aside from one or two narrow pass bands, which in any event is less than the octave. Furthermore, the compensation for the disturbance introduced by the addressing circuit cannot be contemplated in the case of crossed dipoles which are necessary for the measurement of two orthogonal polarizations.